diff --git a/zerver/lib/email_mirror.py b/zerver/lib/email_mirror.py
index 4e9d0b6ee2..8ab1d3e369 100644
--- a/zerver/lib/email_mirror.py
+++ b/zerver/lib/email_mirror.py
@@ -126,7 +126,9 @@ def send_to_missed_message_address(address, message):
     # Testing with basestring so we don't depend on the list return type from
     # get_display_recipient
     if not isinstance(display_recipient, six.string_types):
-        display_recipient = ','.join([user['email'] for user in display_recipient])
+        recipient_str = ','.join([user['email'] for user in display_recipient])
+    else:
+        recipient_str = display_recipient
 
     body = filter_footer(extract_body(message))
     body += extract_and_upload_attachments(message, user_profile.realm)
@@ -139,7 +141,7 @@ def send_to_missed_message_address(address, message):
         recipient_type_name = 'private'
 
     internal_send_message(user_profile.email, recipient_type_name,
-                          display_recipient, subject, body)
+                          recipient_str, subject, body)
 
 
 ## Sending the Zulip ##
diff --git a/zerver/lib/notifications.py b/zerver/lib/notifications.py
index f48def71cb..42ec0734e0 100644
--- a/zerver/lib/notifications.py
+++ b/zerver/lib/notifications.py
@@ -1,5 +1,7 @@
 from __future__ import print_function
-from typing import Any, Tuple, Iterable, Optional
+
+from six import text_type
+from typing import cast, Any, Iterable, Mapping, Optional, Sequence, Tuple
 
 import mandrill
 from confirmation.models import Confirmation
@@ -46,7 +48,7 @@ def one_click_unsubscribe_link(user_profile, endpoint):
     return "%s/%s" % (base_url.rstrip("/"), resource_path)
 
 def hashchange_encode(string):
-    # type: (str) -> str
+    # type: (text_type) -> text_type
     # Do the same encoding operation as hashchange.encodeHashComponent on the
     # frontend.
     # `safe` has a default value of "/", but we want those encoded, too.
@@ -54,20 +56,20 @@ def hashchange_encode(string):
         string.encode("utf-8"), safe="").replace(".", "%2E").replace("%", ".")
 
 def pm_narrow_url(participants):
-    # type: (List[str]) -> str
+    # type: (List[text_type]) -> text_type
     participants.sort()
-    base_url = "https://%s/#narrow/pm-with/" % (settings.EXTERNAL_HOST,)
+    base_url = u"https://%s/#narrow/pm-with/" % (settings.EXTERNAL_HOST,)
     return base_url + hashchange_encode(",".join(participants))
 
 def stream_narrow_url(stream):
-    # type: (str) -> str
-    base_url = "https://%s/#narrow/stream/" % (settings.EXTERNAL_HOST,)
+    # type: (text_type) -> text_type
+    base_url = u"https://%s/#narrow/stream/" % (settings.EXTERNAL_HOST,)
     return base_url + hashchange_encode(stream)
 
 def topic_narrow_url(stream, topic):
-    # type: (str, str) -> str
-    base_url = "https://%s/#narrow/stream/" % (settings.EXTERNAL_HOST,)
-    return "%s%s/topic/%s" % (base_url, hashchange_encode(stream),
+    # type: (text_type, text_type) -> text_type
+    base_url = u"https://%s/#narrow/stream/" % (settings.EXTERNAL_HOST,)
+    return u"%s%s/topic/%s" % (base_url, hashchange_encode(stream),
                               hashchange_encode(topic))
 
 def build_message_list(user_profile, messages):
@@ -142,13 +144,14 @@ def build_message_list(user_profile, messages):
                 'content': [build_message_payload(message)]}
 
     def message_header(user_profile, message):
-        # type: (UserProfile, Message) -> Dict[str, str]
+        # type: (UserProfile, Message) -> Dict[str, text_type]
         disp_recipient = get_display_recipient(message.recipient)
         if message.recipient.type == Recipient.PERSONAL:
             header = "You and %s" % (message.sender.full_name)
             html_link = pm_narrow_url([message.sender.email])
             header_html = "<a style='color: #ffffff;' href='%s'>%s</a>" % (html_link, header)
         elif message.recipient.type == Recipient.HUDDLE:
+            assert not isinstance(disp_recipient, text_type)
             other_recipients = [r['full_name'] for r in disp_recipient
                                     if r['email'] != user_profile.email]
             header = "You and %s" % (", ".join(other_recipients),)
@@ -156,6 +159,7 @@ def build_message_list(user_profile, messages):
                                        if r["email"] != user_profile.email])
             header_html = "<a style='color: #ffffff;' href='%s'>%s</a>" % (html_link, header)
         else:
+            assert isinstance(disp_recipient, text_type)
             header = "%s > %s" % (disp_recipient, message.subject)
             stream_link = stream_narrow_url(disp_recipient)
             topic_link = topic_narrow_url(disp_recipient, message.subject)
@@ -303,7 +307,7 @@ def do_send_missedmessage_events(user_profile, missed_messages, message_count):
         # If we have one huddle, set a reply-to to all of the members
         # of the huddle except the user herself
         disp_recipients = [", ".join(recipient['email']
-                                for recipient in get_display_recipient(mesg.recipient)
+                                for recipient in cast(Sequence[Mapping[str, Any]], get_display_recipient(mesg.recipient))
                                     if recipient['email'] != user_profile.email)
                                  for mesg in missed_messages]
         if all(msg.recipient.type == Recipient.HUDDLE for msg in missed_messages) and \
diff --git a/zerver/lib/test_helpers.py b/zerver/lib/test_helpers.py
index 13ec410ec4..c4fae481f6 100644
--- a/zerver/lib/test_helpers.py
+++ b/zerver/lib/test_helpers.py
@@ -1,5 +1,5 @@
 from __future__ import absolute_import
-from typing import Any, Callable, Generator, Iterable, Tuple, Sized, Union, Optional
+from typing import cast, Any, Callable, Generator, Iterable, Tuple, Sized, Union, Optional
 
 from django.test import TestCase
 from django.template import loader
@@ -292,7 +292,7 @@ class AuthedTestCase(TestCase):
             user_profile=user_profile,
             active=True,
             recipient__type=Recipient.STREAM)
-        return [get_display_recipient(sub.recipient) for sub in subs]
+        return [cast(text_type, get_display_recipient(sub.recipient)) for sub in subs]
 
     def send_message(self, sender_name, raw_recipients, message_type,
                      content=u"test content", subject=u"test", **kwargs):
diff --git a/zerver/models.py b/zerver/models.py
index a7c162fec1..125d4e34ce 100644
--- a/zerver/models.py
+++ b/zerver/models.py
@@ -50,16 +50,16 @@ STREAM_NAMES = TypeVar('STREAM_NAMES', Sequence[text_type], AbstractSet[text_typ
 
 # Doing 1000 remote cache requests to get_display_recipient is quite slow,
 # so add a local cache as well as the remote cache cache.
-per_request_display_recipient_cache = {} # type: Dict[int, List[Dict[text_type, Any]]]
+per_request_display_recipient_cache = {} # type: Dict[int, List[Dict[str, Any]]]
 def get_display_recipient_by_id(recipient_id, recipient_type, recipient_type_id):
-    ## type: (int, int, int) -> Union[text_type, List[Dict[text_type, Any]]]
+    # type: (int, int, int) -> Union[text_type, List[Dict[str, Any]]]
     if recipient_id not in per_request_display_recipient_cache:
         result = get_display_recipient_remote_cache(recipient_id, recipient_type, recipient_type_id)
         per_request_display_recipient_cache[recipient_id] = result
     return per_request_display_recipient_cache[recipient_id]
 
 def get_display_recipient(recipient):
-    ## type: (Recipient) -> Union[text_type, List[Dict[text_type, Any]]]
+    # type: (Recipient) -> Union[text_type, List[Dict[str, Any]]]
     return get_display_recipient_by_id(
             recipient.id,
             recipient.type,
@@ -76,7 +76,7 @@ def flush_per_request_caches():
 @cache_with_key(lambda *args: display_recipient_cache_key(args[0]),
                 timeout=3600*24*7)
 def get_display_recipient_remote_cache(recipient_id, recipient_type, recipient_type_id):
-    ## type: (int, int, int) -> Union[text_type, List[Dict[text_type, Any]]]
+    # type: (int, int, int) -> Union[text_type, List[Dict[str, Any]]]
     """
     returns: an appropriate object describing the recipient.  For a
     stream this will be the stream name as a string.  For a huddle or
@@ -962,6 +962,7 @@ class Message(ModelReprMixin, models.Model):
         if recipient_type == Recipient.STREAM:
             display_type = "stream"
         elif recipient_type in (Recipient.HUDDLE, Recipient.PERSONAL):
+            assert not isinstance(display_recipient, text_type)
             display_type = "private"
             if len(display_recipient) == 1:
                 # add the sender in if this isn't a message between
diff --git a/zerver/tests/test_subs.py b/zerver/tests/test_subs.py
index 7d74b901f4..66293ac9b5 100644
--- a/zerver/tests/test_subs.py
+++ b/zerver/tests/test_subs.py
@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
 from __future__ import absolute_import
 
-from typing import Any, Dict, List, Optional
+from typing import Any, Dict, List, Mapping, Optional, Sequence
 
 from zerver.lib import cache
 
@@ -974,6 +974,10 @@ class SubscriptionAPITest(AuthedTestCase):
         self.assertEqual(msg.content, expected_msg)
         recipients = get_display_recipient(msg.recipient)
         self.assertEqual(len(recipients), 1)
+        assert isinstance(recipients, Sequence)
+        assert isinstance(recipients[0], Mapping)
+        # The 2 assert statements above are required to make the mypy check pass.
+        # They inform mypy that in the line below, recipients is a Sequence of Mappings.
         self.assertEqual(recipients[0]['email'], invitee)
 
     def test_multi_user_subscription(self):